This invention relates to synchro to digital tracking converters in general and more particularly to an improved synchro to digital tracking converter which contains fewer components than those of the prior art and yet has a higher intrinsic accuracy.
Synchro to digital tracking converters are used most commonly to accept analog synchro information and translate that information into a digital format which can be understood by a digital computer. The net result of this translation is the ability of a computer to, for example, interrogate a synchro to determine the angular position of its shaft. A tracking converter differs from the other types of converters, i.e., successive approximation and sampling, in that there is no minimum conversion time required to generate the angular information. Furthermore, tracking converters most commonly have a feedback loop which simulates a Type II servo loop, which allows it to track a constantly changing input with no lag errors. (The velocity constant approaches infinity until the maximum tracking speed is reached.)
Basic to all tracking converters is the ability to accurately generate a steering voltage whose magnitude and phase contains information which causes the Type II control loop to null itself when the digital output angle .beta. is representative of the analog input information .theta.. Most commonly, the steering voltage is proportional to sin (.theta.-.beta.) because this expression does null itself as .theta. approaches .beta..
Most commonly, the function which is actually implemented is the trigonometric expression: EQU sin (.theta.-.beta.).noteq. sin .theta. cos .beta.- cos .theta. sin .beta..
Sin .theta. and cos .theta. are given analog inputs to the converter. They are either provided directly, when the inputs are four wire resolver signals, or are generated by standard Scott "T" transformers from three wire synchro inputs. In order to implement the expression, it is necessary to generate information representing sin .beta. and cos .beta..
The non-linear functions sin .beta. and cos .beta. are generated from the linear digital output angle .beta. by means of an approximation which forms the "heart" of the converter.
Prior state of the art converters most typically generated the sin .beta., cos .beta. approximation by use of two sets of precision ladder networks and two sets of switches.
Information was generated over a full quadrant )0.degree.to 90.degree.) and quadrant switching was required to artificially maintain the information in the first quadrant. Furthermore, a commonly used approximation was that: ##EQU1## where K.sub.1 is the best fit constant from 0.degree. to 90.degree., and is equal to 0.555R.
N is a running variable from 0 to 1 as the output angle .beta. varies from 0.degree. to 90.degree.. This approximation is accurate to within .+-.1.8 arc minutes over the quadrant, when evaluated as a tangent function, i.e., when tan .theta.=(sin .theta./cos .theta.).noteq.(sin .beta./cos .beta.)= tan .beta.. Since the end item accuracy is most typically four arc minutes, this inherent error is a significant portion of the total error budget.
The use of two ladder networks and two sets of switches represent duplication of the most costly and critical components in the converter. Furthermore, the impedance of the switch in the most significant bit of the ladder network, with a weight of 45.degree., represented a significant error source, since a 20 ohm error in series with a nominal resistance of 20,000 ohms (i.e., a 0.1% error) would be an error source of 1.35 arc minutes.
Thus, it can be seen that there is a need for a simpler, more accurate synchro to digital tracking converter.